Inter network roaming is considered an essential element of mobile networks, and allows a mobile subscriber via his/her UE to make use of mobile services outside of the radio coverage of the associated home service provider, for example when traveling outside the home country.
Many technologies and solutions exist, which facilitate the roaming between a home network and a visited network. Mobile Network Operators (MNOs) may contract agreements to allow roaming. Accordingly, the MNOs may set up the technical means to realize the roaming either directly, that means from MNO to MNO, or by making use of roaming services which enable roaming through a dedicated roaming infrastructure, which may or may not be operated by a third party or, for example, by an MNO who may offer services in more than one country.
In some scenarios, no such roaming agreements may exist between MNOs, but for exceptional situations a temporary roaming should still be possible. One example is the national roaming for competing MNOs, which normally do not allow subscribers of the competitor's network to roam to their own network. However, assuming an emergency situation, such as a car breaking down in an area with very low or no radio coverage of the home MNO of a given UE, the driver using that UE may have no possibility to use an alternative network, unless the driver maintains multiple subscriptions. On the other hand, the driver may not be allowed to exploit emergency functionalities, since those functionalities might be reserved for more severe emergencies (for example, a state of national emergency, such as an earthquake or a tsunami, or at least states of emergency involving injury to persons).
One reason that national roaming (e.g., if the UE is still in the subscriber's home country but has no or insufficient radio coverage for its designated MNO) may not be allowed resides in the fact that a mobile subscribers with a given UEs might roam accidentally, simply because radio coverage from the home network of the given UE is not as good as radio coverage from non-home networks. As a disadvantage for the designated MNO, that MNO may lose revenue; on the other hand, as a disadvantage for the subscriber, the subscriber may disagree with extra roaming fees liable for using a non-home MNO, or may at least wish to be warned about those extra roaming fees prior to consenting to such a roaming operation.